The present invention relates to the art of blasting, and more particularly to a method for preparation of a water-in-oil type emulsion explosive, having a discontinuous hydrophilic oxidizer phase, containing oxidizing salts, dispersed in a continuous lipophilic fuel phase, containing combustible materials, and being sensitized by voids dispersed in the emulsion.
In the manufacture of this kind of explosives the introduction of the voids presents a number of problems. The size of the voids must be controlled, since too small voids are unable to locally ignite the fuel/oxidizer mixture while too large voids reduce either the number of ignition points or the energy concentration in the explosive as a whole. A homogeneous distribution of the voids is essential since local deficiencies may leave unreacted material after detonation and even cause a termination of the detonation wave if the unsensitized area is large. In general it also is necessary that void structure and distribution are stable over time and resistant to dead pressing and emulsion deformation. The void introduction process itself is complicated by the great component density difference. All these problems will be more pronounced in site manufacture of bulk explosives where condition control cannot reach factory standards, simpler mixing devices have to be used and safety requires late but rapid density reduction.
Several methods are known for introducing voids in emulsion explosives.
Air or other gases can be mechanically worked into the emulsion during or after its manufacture. It is difficult to disintegrate the gas into fine enough bubbles and simple mixing devices are generally not sufficient. Long term stability is affected by partial dissolution of the free gas, by coalescence of bubbles or by escape of gas, especially when working or deforming the emulsion.
Several suggestions have been made for in situ formation of occluded gas in the emulsion by the use of gassing agents, see for example U.S. Pat. Nos. 3,706,607, 3,711,345, 3,713,919, 3,770,552, 3,790,415, and 4,008,108. Common problems with these known methods are difficulties with dosage and distribution of the normally quite small gassing agent additive in the emulsion. Good timing between gassing and mixing is required. In bulk manufacture problems are frequently encountered in timing gassing reaction against charging operation and in halting the reaction at charging interruptions.
Adding cellular or void containing materials in the emulsions has the advantage of isolating the voids from the emulsion matrix whereby durability and mechanical resistance is improved in relation to free gas bubbles. Rapid and simple introduction of these materials in an emulsion matrix is difficult, however, due to the fragile nature of the particles and the tendency of the fine, light and dusty material to resist wetting and entrain an uncontrolled amount of additional air into the emulsion. The U.S. Pat. Nos. 4,310,364 and 4,338,146 disclose manufacturing methods in which cellular particles are added to a salt solution before fuel phase addition. The method requires an extended agitation to convert an oil-in-water emulsion into a water-in-oil emulsion and during a substantial part of the manufacturing process a gas sensitized explosive will be present.